COMPARACIÓN DE LAS HUELLAS HÍDRICAS

In carrying out the physicochemical measurement of liquid phases, it is necessary to have a solid to support the liquid. Ideally, the solid acting as support would behave like a sponge holding the thin hquid film and maintaining the stationary phase in such a state as to enable the flow of gas to pass through it^ without playing any further role in the interaction process. In practice, this is not always true, as observed by James and Martin^ on the publication of their first experimental work on GLC on the obtainment of asymmetrical eluted chromatographic peaks, which was explained as adsorption of the solutes on the surface of support. Adsorption of solute by the support, if substantial, is usually manifested by peak tailing and a decrease of retention volume with sample size. This arises from the presence, on the heterogenous surface, of high energy adsorption sites, which desorp solute only slowly. Such sites produce strong Langmuir-type curvature in the adsorption isotherm, see figure 5.0, because the heat of adsorption is considerably greater than the heat of liquefaction. Isotherm curvature causes retention to vary with concentration and peaks to tail. Support adsorption can still be present even when there is no tailing and retention is constant (an example can be seen with alkanes). Lack of tailing can be due to a linear adsorption isotherm or to a masking of tailing by peak-broadening phenomena. Thus, on unwetted supports the liquid is probably distributed in deep, discontinous patches, giving broad peaks in which any support tailing is masked. Adsorption effects thus are often not observed. Adsorption of solute by the solid support contributes to retention in the majority of GLC systems, but to very different extents, depending on

iina the nature of the solute, liquid phase and support. If adsorption on^ support contributes to retention aside from bulk liquid partitioning, then the retention volume equation is rewritten as;

Vn — KlVl + KsAs (7.0)

Where Vn is the net retention volume, Kl and Vl are the liquid partition coefficient and liquid volume respectively, As is the surface area for the support in the column and Ks is the corresponding adsorption coefficient. Since the coated and uncoated proportions of the surface are difficult to measure separately, Ks and As cannot easily be determined and so are known as the product, KsAs, obtained as residual after subtracting KlVl from Vn.

Kwantes and Rijinder^ studied four possible combinations of polar and non-polar solutes with polar and non-polar phases, polar are: alcohols, aldehydes, ketones, or esters. These authors found that the peaks obtained in these compounds were sloping with pronounced tailing, while the retention time was strongly dependent on the sample size, being narrow with large sample size and vice versa. This is indicative of residual adsorption of the polar compound on the support. It is possible that adsorption on the liquid surface was also contributing to the observed effects, though to a much smaller extent than support adsorption."^

The above observed phenomenon would most likely not occur if the liquid loading on the support is high, as this would completely cover the entire surface of the solid support. This is not entirely true if the liquid phase does not ‘wet’ propertly on the solid support, ie. the manner in which the stationary phase is distributed over the surface of the support is determined by the nature of the interactions and the technique used for coating.

The distribution of the stationary phase on the solid support surface might not be uniform. The liquid phase film may, in general, cover the solid support surface completely or partially, forming separate areas.^ It may have a uniform thickness (even distribution) or the thickness may vary from one microportion to another (unven distribution). This phenomenon seems to hold in the case of poor wettability of the support surface with the stationary phase.

Keulmans,^ who shared the same concept as Van Deemter/ believes that the hquid accumulates in the smallest pores and in the holes under the effect of capillary forces. As the amount of the liquid phase increases, larger pores start to be filled. The filling of the smaller pores first on a solid support with the liquid phase was demonstrated experimentally by Baker et al,* who conducted a porosimetric study of an coated solid support ( of the Chromsorb P type) and a support after coating with different amounts of the liquid phase (5-33%). The results indicated that fine pores are filled first. The liquid film also seems to fih fine pores inhomogenously and to form a thin film over the remainder of the surface of the solid support. Giddings and Saha^ proposed that the liquid phase forms a continous film over the solid support surface; part of it is in capillaries while the remainder coats the walls of larger pores with a continous thin film.

Although diatomite supports are weak adsorbents in comparison with alumina, sihca gel, etc., their activity is usually sufficient for the afore-mentioned effect of adsorption to become apparent. It is less obvious with non-polar solutes, polar liquid phases and high loading of liquid phase covering active portions of the solid’s surface. Most of the liquid phases studied here (as shall be seen later) have fairly low percentage loading on the solid support, the phenomemon of adsorption of solutes onto the surface of solid support as described above is frequently observed in this work. This factor cannot be entirely ignored as adsorption on the solid support could significantly contribute to retention in gas liquid-chromatography.

To gain an insight into the possibility of adsorption effects of the support, a study of gas-solid partition was made. The solid used to support the liquid phases is chromosorb GAW-DMCS, this means that the type of chromsorb is G, acid washed (AW) and had been silanised with dimethylchlorosilane (DMCS), with mesh size of 40-60 or 60-80.

There are a number of supports commercially available; among the most important support materials are the diatomaceous earths, which account for more than 90% of the packed column GLC produced.'® Diatomite (diatomaceous earth, kieselguhr) is

composed of the skeletons of diatoms, single-celled algae, which have accumulated in large beds in various part of the world. The skeletal material is essentially microamorphous silica with small amounts of alumina and metallic oxide impurities. The porous nature of the diatomite with its associated secondary structure gives the material a high surface area to weight ratio, approximately 20 mVg, to allow high liquid phase loading. The support was produced by firing the raw diatomite skeleton at high temperature to strengthen and to agglomerate the particles into suitable mesh size (ranging from 10-20 to 375-400 etc.) with regular shape and narrow range of cross-sectional diamete

high column efficiency.

cu

cross-sectional diameters, to use as,support in gas-liquid chromatography that gives

The treated natural diatomaceous supports still contain metallic impurities and one of the methods used to remove these is by washing with acid and or base. The latter process may remove most of the metalhc impurities but the support is still not inert, due to the silanol groups present on the surface of the solids, as illustrated in figure 7.0. Of the various available silanizing t r e a t me n t s , * t h e most widely used approach to deactivate the silanol groups is to cap the hydroxide moiety with dimethylchlorosiloxane,* which reduces adsorption by 80%, when preceeded by acid washing. Bohemen et al*^ suggested that two reactions are involved. The first involves two adjacent silanol groups:

-S i—O Si + S iC l2(C H p2 ---► S i—O — S i + 2HC1

OH OH O o

CH3/ "CH^

and the second a single silanol group:

I I I I C l

— S i - 0 — S i - O H + S iC l2 ( C H 3 ) 2 ► — S i - 0 — S i— O — S i - C H -h H C l

3

CH3

Usually a methanol wash is employed, in which case the undesirable Si-Cl group found by the second reaction is converted to a methoxy derivative.12

Cl OCH

I I 3

-Si-CH ^ + CH3OH --- ► — Si-C H ^ + HCl

CH3 CH3

Figure 7.1 The conversion to a methoxy derivative

Silanisation of the support changes the hydrophihc character to hydrophobic and, consequently, it is no longer completely wetted by polar liquid phase. So, preparation for polar liquid phases packings on silanised support is quite difficult. Recommendations have been made to improve the support wetting characteristics for diatomaceous support by nonpolar phases surface treatment with cychc siloxanes’^ or silanisation with bulky reagents such as octadecyldimethylchlorosilane. ^ ^ These treatments have indicated some improvement in the support wetting by providing higher column efficiency and improved mass transfer characteristics, however, these treatments do not yield ideal conditions for non-polar phases. However, the experiences gained from a few of the polar phases in this work has shown little trouble with coating on silanised support. An other way of deactivating the reactive support surface is by precoating the support with a non-extractable film of carbowax 20 M or polyester; this has been shown to be more appropriate than silanisation procedures for polar phases.

The most extensive chemical treatment and silanisation of diatomaceous support still does not completely deactivate those active centres which cause tailing of strongly basic or acidic components. When compounds of this type are analyzed or separated, the addition of small quantities of “tailing reducers” can be used. Tailing reducers are coated onto the support in a manner similar to that for the liquid phase and, to be

effective, they must be stronger acids or bases than the compounds to be chromatographed. For amines, the tailing reducers could be a few percentage of potassium hydroxide or polyethyleneimine.^^’^® For acidic compounds, a suitable tailing reducer is phosphoric acid. It must be remembered that these active substances will also act as subtractive agents and thus an acidic tailing reducer will remove basic substances from the chromatogram and vice versa. The phase itself must also be compatible with the reagents. For example, potassium hydroxide and phosphoric acid catalyse the depolymerisation of polyesters and polysiloxanes. The “tailing reducers” are not appropiate to apply in the case where the stationary phase is under study, ie. inverse gas chromatography.

Scanning electron microscope (SEM) has been used to study the surface of a number of supports.^^'^^ The electron micrograph of natural support, figure 7.2, showed the ruggedness of the supjprt, and in figure 7.3, a coated layer of DMCS on the surface of the bare support showed a smoother appearance of the natural support.

There are different types of chromosorb on the market; each has been prepared differently and thus has different properties, chromosorb G, P, W, A, T, 101 etc. The chromsorb G used here is a specially prepared support; it is hard, robust, and has an oyster white appearance. Chromosorb G has a low loading capacity but is more dense than the ordinary white supports: For example in a given column, there is approximately 2.5 times the weight amount of chromsorb G as chromosorb W, and therefore, 2.5 times the liquid phase, of the same nominal percentage weight per weight coating.

»

F ig u re 7.2 C h ro m o so rb G , a c id w ashed,"^ 1500x